Now more than ever, electric utility power distribution systems are being constructed underground. Underground systems pose new operational and maintenance challenges by virtue of being largely unseen. In response to these challenges, organizations such as the Institute of Electrical and Electronics Engineers (IEEE) and the American National Standards Institute (ANSI) have implemented standards and codes to insure operating personnel safety and proper system performance. One such standard recommends the grounding (i.e., shielding) of individual underground distribution system components at multiple system points (e.g., cable splices, transformers, switches). Grounding system components (or their enclosures) helps eliminate accessibility to hazardous voltages by operating personnel.
Fuses are well known for use in power distribution systems for reliable interruption of fault current where reclosing is not required. When used in underground applications such as direct burial, switchgear, or vaults where there is a high probability of submersion, it is desirable for fuses to be compact and enclosed or encapsulated in electrically insulating, high dielectric strength material. To ground an underground fuse in order to protect personnel from hazardous voltages, the entire exterior must be conductive, producing a ground plane thereon. As a result, steep voltage gradients throughout the insulating material of the fuse are formed. The high system voltages present in the fuse are separated from the ground plane by a relatively thin insulating material. Under these conditions there is a tendency for the fuse to become electrically stressed and corona to discharge or arc within the fuse (e.g., discharge through the insulating material from the high voltage fusible element to the exterior ground plane). After the fuse has been subjected to such corona discharge for a long period of time, the fusible elements can be damaged and may not operate properly under short circuit or fault-interrupting conditions.
In order to mitigate corona discharge within the fuse, high voltage stress to the fusible elements must be eliminated. One established method to eliminate the high voltage stress inside the fuse is to envelope the fuse with a conductive surface that is at the same potential as the fusible element. This method of enveloping the fuse finds support in the Faraday Cage theory in which a conductive enclosure acts as a shield against electric fields and electromagnetic waves. Previous attempts to enclose the fuse have focused on applying a conductive or semiconductive coating such as paint to the fuse exterior surface. Although the applied coating may help eliminate voltage stress, often the coating provides fault current with a secondary conductive path (e.g., flashover) during a “blown” fuse condition thereby rendering the fuse useless.
Effective elimination of corona in encapsulated fuses for power distribution systems has been elusive. In view of the foregoing, it would be desirable to provide an encapsulated fuse that resists both corona discharge and flashover.